Original article

Retrospective investigation of the remaining teeth status of patients with

implant-supported fixed partial dentures in unilateral free-end edentulism

Seiya Yamazaki DDS, PhD, Hikaru Arakawa DDS, PhD, Kenji Maekawa DDS, PhD,Emilio Satoshi Hara DDS, Kinji Noda DDS, PhD, Hajime Minakuchi DDS, PhD,

Wataru Sonoyama DDS, PhD, Yoshizo Matsuka DDS, PhD, Takuo Kuboki DDS, PhD*

Department of Oral Rehabilitation and Regenerative Medicine, Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences,

2-5-1 Shikata-cho, Okayama 700-8525, Japan

Received 14 May 2013; received in revised form 8 July 2013; accepted 9 August 2013

Available online 11 October 2013

Abstract

Purpose: This study aimed to compare the various complication-free rates and survival rates of remaining teeth among implant-supported fixed

dentures (IFDs), removable partial dentures (RPDs) and no-restoration (NR) patients with unilateral free-end edentulism.

Material and methods: The study subjects were selected among those who received prosthodontic treatment at Okayama University Dental

Hospital for their unilateral free-end edentulism (2 or 3 missing teeth). Thirty-three patients were included in the IFD group, 41 matched patients in

the RPD group, and 10 patients who received RPDs but refused their use were regarded as NR group. The remaining dentition was classified into

five subcategories in relation to the missing portion: adjacent teeth to the missing portion (AD), contralateral posterior dentition in the same jaw

(CS) and in the opposite jaw (CO), ipsilateral opposing posterior dentition (IO), and anterior dentition (AN). Complications were defined as tooth

extraction, periodontal lesions, periapical lesions or loss of retention of the prosthesis and were assessed by one examiner based on the hospital

chart records.

Results: The cumulative complication-free rates in the remaining teeth were significantly different among each of the three groups ( p < 0.01),

with a significantly lower incidence rate in the IFD group. Regarding the cumulative survival rate of the remaining teeth, there was a significant

difference only between IFD and NR group ( p = 0.01), especially in the CO region ( p = 0.04).

Conclusions: Stable posterior occlusal support obtained with IFD treatment for unilateral free-end edentulism may reduce the incidence of

complications in the remaining teeth, by decreasing the adverse mechanical stress.

# 2013 Published by Elsevier Ireland on behalf of Japan Prosthodontic Society.

Keywords: Implant-supported fixed dentures; Unilateral free-end edentulism; Complication-free rate; Survival rate; Remaining dentition

www.elsevier.com/locate/jpor

Available online at www.sciencedirect.com

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Journal of Prosthodontic Research 57 (2013) 262–267

1. Introduction

The main goals of prosthodontic care are to rehabilitate oral

function and to preserve remaining teeth. However, traditional

prosthodontic modalities (e.g., fixed- or removable-partial

dentures) have been speculated to increase the risks for

abutment tooth loosening due to excessive occlusal forces or

torqueing forces from the adjacent edentulous space [1–3].

These bending forces on the abutment or adjacent teeth have

been shown to differ according to the prosthodontic modality,

with a poorer survival rate in the cases restored with

* Corresponding author. Tel.: +81 86 235 6680; fax: +81 86 235 6684.

E-mail address: kuboki@md.okayama-u.ac.jp (T. Kuboki).

1883-1958/$ – see front matter # 2013 Published by Elsevier Ireland on behalf o

http://dx.doi.org/10.1016/j.jpor.2013.08.001

removable-partial denture (RPD), compared to fixed partial

dentures (FPDs) [4,5].

On the other hand, implant-supported denture (IFD), which

stands alone, has been speculated to protect teeth adjacent to

the edentulous space. Although previous studies have

examined the survival of implant fixtures [6,7], those outcomes

have not fully evaluated the impact of IFD treatment on the

remaining teeth in comparison with other treatment options.

Therefore, we conducted a series of studies that separately

evaluated the prognosis of the remaining teeth adjacent to

different types of edentulous spaces (bounded edentulous,

unilateral distal edentulous and large bounded edentulous)

treated with IFDs. The first report assessed the complications

rate of natural teeth adjacent to IFDs to those teeth serving as

abutments for FPDs [8].

f Japan Prosthodontic Society.

Fig. 1. The method of selecting patients for each treatment group and the time

line of the observation and outcome variables of this study.

S. Yamazaki et al. / Journal of Prosthodontic Research 57 (2013) 262–267 263

This second retrospective cohort study compared the various

complication-free rates of the remaining teeth among IFDs,

RPDs and no restoration (NR) in patients with unilateral free-

end edentulism. Survival rates of the remaining teeth among

these three groups were also evaluated. The null hypothesis was

that no significant difference in complication-free and survival

rates would be observed among the three treatment modalities.

2. Materials and methods

2.1. Study population

Three cohorts of subjects were included in this retrospective

study. Intended sample was the patient population seeking

treatment for their unilateral free-end edentulism (2 or 3

missing teeth) at the Fixed Prosthodontic Clinic of Okayama

University Hospital, Okayama, Japan. The IFD group

consisted of a consecutive series of 53 patients who were

treated between February 1990 and March 2002. However,

twenty patients were excluded because they received addi-

tional IFDs or RPDs for other missing portion, which left a total

of 33 patients (male/female: 9/24, mean age: 53.8 � 11.7

years) in this IFD group.

The RPD group of 41 patients (male/female: 8/33, mean age:

55.8 � 9.4 years) was exhaustively selected from the 422

consecutive RPD patients treated between April 1997 and

March 2005. They comprised a matched group based upon

gender, age, missing portion, and functional duration of the

prosthesis with those of the IFD group, as determined from the

dental and laboratory records. Ten patients who had completed

the treatment but did not tolerate wearing and stopped using the

RPDs for their unilateral free-end edentulism within one month

after the insertion were regarded as NR group (male/female: 7/

3, mean age: 52.3 � 9.2 years) (Fig. 1). This study protocol was

reviewed and approved by the Ethical Committee for Human

Research in Okayama University Graduate School of Medicine,

Dentistry and Pharmaceutical Sciences (#213).

2.2. Classification of tooth complications or survival

The outcome variable of this study was incidence of

complications in the remaining teeth. The condition of the

remaining teeth was assessed by one investigator (S.Y.) from

the records in the hospital chart. Complications were defined as

tooth extraction, periodontal lesions, periapical lesions or loss

of retention of the prosthesis. Diagnoses of periodontal and

periapical lesions were based on clinical examination; and

when symptomatic radiographic examinations were also

performed. Complications were investigated in the remaining

dentition, but separately in five sub-regions: adjacent teeth to

the missing portion (AD), contralateral posterior dentition in

the same jaw (CS) or in the opposite jaw (CO), ipsilateral

opposing posterior dentition (IO), and anterior dentition (AN)

(Fig. 2).

In the IFD and RPD groups, the observation started at the

insertion date of the final restoration, whereas in the NR group,

the observation started when the patients reported not to be

using the RPDs, and the observation finished on March 31,

2007 (Fig. 1). Patients’ follow-up visits were scheduled at

least every six months, and the treating dentist checked the

status of all restorations, the periodontal condition and oral

hygiene quality determined by presence of biofilm. If the oral

hygiene quality of the patients was unsatisfactory, additional

instruction was performed at the follow-up visit. In addition,

relining of the RPD was performed when the attending

doctor judged necessary for the subjects who were wearing

RPDs.

The primary endpoint was established when any of the

aforementioned complications was described in the dental

records. If there happened a primary incidence of tooth

complication in one category, the observation was then

terminated. Patients who did not show up to the examination

within two years prior to the end of the study were regarded as

censored cases, to whom the complication-free period was

established to be from the date of final treatment completion to

the last follow-up visit.

In addition to the analysis of tooth complications, the

survival rate of the remaining teeth was also evaluated.

Analysis of survival rates included the cases in which the

remaining tooth was not extracted during the observation

period. The methodology of observation and data analysis were

the same for the analysis of complication-free rates and survival

rates.

Fig. 2. Classification of remaining dentition. The remaining dentitions were classified into five categories in relation to their missing portion: adjacent teeth of the

missing portion (AD), the contralateral posterior dentition in the same jaw (CS) and in the opposite jaw (CO), ipsilateral opposing posterior dentition (IO), and

anterior dentition (AN).

Table 1

Base-line comparison of the IFD, RPD and NR groups.

IFD group RPD group NR group p-Value

n = 33 n = 41 n = 10

Male/female 9/24 8/33 3/7 0.655b

Mean age (y) 53.8 � 11.7 55.8 � 9.4 52.3 � 9.2 0.522a

Missing unit

Two/three 22/11 27/14 8/2 0.680b

Maxilla/mandible 1/32 2/39 6/4 <0.001b

Remaining teeth (tooth n) 23.8 � 2.0 23.1 � 2.4 23.7 � 1.8 0.377a

Functional duration (y) 6.6 � 4.1 5.0 � 3.4 6.0 � 4.1 0.194a

a One-way factorial ANOVA.b x2-test (mean � SD).

Fig. 3. Cumulative complication-free rate of the whole remaining teeth. The

results of the log-rank test revealed that there were significant differences

among the IFD, RPD and NR groups in cumulative complication-free rate of the

whole remaining teeth (between IFD and RPD; p = 0.01, between IFD and NR;

p < 0.01).

S. Yamazaki et al. / Journal of Prosthodontic Research 57 (2013) 262–267264

2.3. Statistical analysis

Chi-square test and one-way factorial ANOVA were used to

compare baseline data among IFD, RPD, and NR groups

regarding gender, age, missing unit, remaining teeth number,

and functional duration of the prosthesis. Complication-free

and survival rates were calculated by Kaplan–Meier analysis

[9]. The log-rank test was used to compare the survival curves

among the three groups [10]. The level of significance was set

as p < 0.05.

3. Results

3.1. Baseline data

As shown in Table 1, comparison of baseline data revealed

no significant statistical differences among IFD, RPD, and NR

groups in regard to gender ratio, mean age at prosthesis

insertion, number of missing teeth at intended missing units,

number of remaining teeth, or functional duration of the

prosthesis. However, there was a significant baseline difference

in regard to the site of missing units among the three groups

(maxilla/mandible: 1/32 for IFD group, 2/39 for RPD group, 6/

4 for NR group; p < 0.001).

3.2. Complication-free and survival rates

This retrospective cohort study revealed that there were

significant differences among the IFD, RPD and NR groups in

the cumulative complication-free rate of the whole remaining

teeth (between IFD and RPD; p = 0.01, between IFD and NR;

p < 0.01) (Fig. 2). The IFD group presented a significantly

higher cumulative complication-free rate of remaining teeth

compared to RPD and NR groups, particularly in the CO and

AN regions (Fig. 3). The incidence of complications in the

remaining teeth in the IFD group was also lower than in the

Fig. 4. Cumulative complication-free rates of the remaining teeth in 5 categories. The results of the log-rank test indicated that cumulative complication-free rate of

remaining teeth in the IFD group was significantly higher than the other two groups in the CO and AN region.

Fig. 5. The incidence of remaining tooth complications in each group. The

incidence of remaining tooth complications in the IFD group was also lower

than the RPD and NR groups (IFD, RPD and NR were 42%, 59% and 90%,

respectively).

S. Yamazaki et al. / Journal of Prosthodontic Research 57 (2013) 262–267 265

RPD and NR groups (IFD, RPD and NR were 42%, 59% and

90%, respectively: Fig. 4). However, no significant difference

was observed in the cumulative complication-free rate of the

whole remaining teeth regarding the tooth position, i.e.,

between maxillary and mandibular teeth (Fig. 5).

Regarding the cumulative survival rate of the whole

remaining teeth, there was a significant difference between

IFD and NR group ( p = 0.01) especially in the CO region

( p = 0.04), whereas no difference could be observed between

the IFD and RPD groups (Figs. 6–8).

4. Discussion

To the best of our knowledge, this study was the first study to

compare the complication-free and survival rates of the

remaining teeth in patients treated with IFD, RPD or NR for

their unilateral free-end edentulism. The results of the Kaplan–

Meier analysis indicated lower complication rates in the IFD

treated group, which demonstrated the protective effect of IFD

treatment on the remaining teeth compared to RPD and NR

groups. Particularly, the low complication rates in CO and AN

regions indicate that IFD treatment may not only preserve

adjacent teeth to the unilateral free-end edentulism, but also

those in the contralateral posterior and anterior regions,

possibly by giving stable posterior support that reduces the

mechanical overload to contralateral posterior teeth and avoids

non-axial forces to the anterior teeth.

Regarding the survival rate of the remaining teeth, NR group

presented a significantly lower survival rate compared to that of

IFD group; particularly in the CO region (Figs. 6 and 7). On the

other hand, there was no significant difference between RPD

and NR groups. These results are in agreement with previous

reports, which showed no significant difference in the survival

rates of remaining teeth between RPD for molar replacement

and no replacement (shortened dental arch: SDA) [11].

Regarding the fact that more complications were observed in

the CO region of NR group (compared to IFD group), we could

not discard the possibility that it was merely a coincident

finding mainly due to the low number of subjects in this study,

or whether there was an excessive occlusal loading due to a

possible unilateral molar mastication, which could aggravate

the periodontal condition in the remaining teeth at the CO

region, and eventually induce more tooth loss. This hypothesis

is supported by the high prevalence of periodontal lesions in NR

group. Future research with larger sample size will be

promising to clarify this issue.

One interesting finding was that there were no significant

differences in complication-free or survival rates at the AD and

CS regions among the three groups. Previous research findings

have reported that the abutment teeth of RPD have higher risk

of failure than other treatment modalities. Miyamoto et al.

evaluated the failure of teeth relative to their treatment history

Fig. 7. Cumulative survival rate of the whole remaining teeth. The results of the

log-rank test revealed that there was a significant difference between IFD and

NR group ( p = 0.01), but no difference was observed between the IFD and RPD

groups.

Fig. 8. Cumulative survival rates of the remaining teeth in 5 categories. The results of the log-rank test indicated that cumulative survival rate of remaining teeth in the

IFD group was significantly higher than the NR group in the CO region ( p = 0.04).

Fig. 6. The cumulative complication-free rate of the whole remaining teeth

compared between maxilla and mandibular teeth. Significant difference was not

observed between the jaws.

S. Yamazaki et al. / Journal of Prosthodontic Research 57 (2013) 262–267266

[12]. They collected a longitudinal follow-up data (from 15 to

23 years) of 3071 teeth from 148 patients and treatment history

were categorized at baseline as (1) sound, untreated tooth, (2–5)

1–4+ -surface restoration, (6) complete crown, (7) abutment for

FPD, (8) abutment for RPD, and (9) root canal treatment. The

results of that study demonstrated that the only teeth with an

increased risk for failure resulting in an eventual extraction

were RPD abutments (relative risk = 5.5). In addition, Bergman

et al. reported that 44% of the abutment teeth needed a

restorative treatment after 10 years [13]. Furthermore,

Vermeulen et al. analyzed the failure of the abutment teeth

in 886 RPDs worn by 748 patients. The results of that study

showed that approximately 40% of the conventional RPDs

survived 5 years and over 20% survived 10 years [14]. On the

other hand, the results of this present study did not show the

differences in complication-free and survival rates of teeth in

AD and CS regions among the three groups. One possible

explanation for these results is that the edentulous area was

limited to 2 or 3 teeth and, therefore, the torqueing forces from

the RPD onto the abutment teeth during mastication could be

relatively weak compared to those of the above previous

studies. Future investigations are needed to compare the

prognosis of the remaining teeth in patients treated with IFDs or

RPDs for restoration of large edentulous spaces.

Since this study was conducted as a retrospective cohort

study, there were several shortcomings and limitations

involved. First, the sample size of this study was small. To

reach more concrete conclusions, future studies with

S. Yamazaki et al. / Journal of Prosthodontic Research 57 (2013) 262–267 267

well-controlled larger sample size are needed. In addition, there

is a possibility that the results of this study could be affected by

factors that were unfortunately not included in this investigation,

such as initial baseline differences among the three groups in

regards to the periodontal condition. Oppositely to other

prosthetic therapies, implant treatment essentially involves a

surgical procedure, and it is problematic to employ a randomized

controlled trial design for comparisons of treatment effects

between implant therapy and other therapeutic options for

edentulism. Thus, future prospective cohort studies are desirable

with adjustment of candidate risk factors such as baseline of

periodontal condition and bite force of each treatment group.

5. Conclusions

The cumulative complication-free rates of the whole remain-

ing teeth were significantly different among the IFD, RPD and NR

groups. Moreover, the cumulative complication-free rate of the

remaining teeth in the IFD group was significantly higher than the

other two groups in the CO and AN regions. Regarding the

cumulative survival rate of the whole remaining teeth, there was a

significant difference between IFD and NR group, especially in

the CO region; but no difference was observed between the IFD

and RPD groups. With the limitation of the study due to the small

sample size and the lack of the examination of periodontal

baseline condition, these results suggest the possibility that the

stable vertical support gained with IFD treatment for unilateral

free-end edentulism reduces adverse mechanical stress and

consequently preserves the remaining teeth.

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